The present invention relates in general to semiconductor devices and, more particularly, to providing a low cost, high channel density, trench gate transistor.
Trench power metal oxide semiconductor field effect transistor (MOSFET) devices are used in many applications including power supplies, battery chargers, computers, and cell phones. An important characteristic of a trench power MOSFET device is its on-resistance (RDS(ON)). For a given device area, it is desirable to minimize the RDS(ON) of the power MOSFET. Trench power MOSFETs are typically fabricated using a large number of cells. Each individual cell is itself a small trench gated transistor. By connecting these cells in parallel, it is possible to achieve a high current carrying capability, and low RDS(ON) for the device. A typical trench power MOSFET typically has between 1,000 and 1,000,000 cells. Each cell comprises a gate structure in a trench, a diffused base region between adjacent trenches for providing a channel, a diffused source region adjacent to each trench and the top surface, and a base contact region at the top surface. The base contact region in the cell is necessary in order to avoid a floating base condition, which would adversely affect the breakdown voltage and the avalanche energy capability of the device.
One way to achieve a lower RDS(ON) for the device is to increase the channel density, which is the effective width of channel per unit area of the device. The cell density typically is increased by decreasing the dimensions of each cell. However, as the cell dimensions shrink, the width of the source region becomes smaller and more difficult to manufacture using low cost photolithography equipment. Thus, to reduce the cell size, more expensive and complex photolithographic equipment and processes are required to maintain control over the cell dimensions to avoid introducing defects that adversely effect transistor performance.
The more expensive lithographic equipment and complex processes previously needed to achieve a small cell size result in increasing the overall cost of fabricating the power MOSFET device.
Hence, there is a need for a power trench MOSFET semiconductor device having a small cell size and a method of making this device at low cost.